1. Field of the Invention
The present invention relates generally to high efficiency power converters for use in supplying a wide range of load current, and relates more particularly to a DC-DC converter that avoids operation in an audible frequency range when supplying loads current.
2. Description of Related Art
High efficiency demands are often placed on power converters, especially DC-DC converters for use in a number of applications. One particular application that produces a broad range of load current demand conditions is in the area of portable equipment products, such as notebook computers. The power demands of portable equipment can change dramatically from moment to moment, due to the focus on power efficiency, extending battery life and reducing power consumption. A number of solutions for portable equipment using DC-DC converters have been proposed, typically focusing on efficiency and handling dramatic changes in load current demands. For example, one way to handle rapidly changing load current demands, while maintaining high efficiency is to skip clock pulses or change switching frequency of the DC-DC converter as a function of load current. As load current demand decreases, more pulses are skipped, or frequency is further reduced resulting in a lower amount of output current.
As apparent switching frequency decreases, it is possible to enter an audible frequency range, resulting in the production of audible output from components of the DC-DC converter. In particular, output components such as inductors or capacitors can be driven at an audible frequency, resulting in audible buzzing or ringing of the components which is highly undesirable in generally, and particularly undesirable in the case of portable equipment.
Referring to FIG. 1, a circuit 10 illustrates the operation of a DC-DC converter according to a conventional design that presents challenges related to operation in the audible frequency range. Circuit 10 is generally efficient in continuous conduction mode related to high current demand through the operation of switches M1, M2 configured in a switching half bridge arrangement. In the case of low current demand, circuit 10 operates in discontinuous conduction mode, and can still maintain a high efficiency as the frequency decreases in a proportional relationship to the load current demand. FIG. 2 illustrates voltage and current weight forms for low current demand with a low switching frequency and discontinuous mode.
As the switching frequency of circuit 10 continues to decrease, it can enter the audible frequency range producing audible sound in external components such as inductor Lx or capacitor Cout. However, if the switching frequency range is limited to be above the audible frequency range, an over voltage condition may be generated where circuit 10 supplies a greater current than is demanded by the load. If the additional current output is shunted, the efficiency of circuit 10 decreases dramatically.
It would be desirable to obtain a DC-DC converter for portable equipment that does not suffer from the drawbacks of the conventional art.